Following the publication of a three-phase time sensitive model by Weisfeldt and Becker (Weisfeldt M L, Becker L B: “Resuscitation after cardiac arrest. A 3-phase time-sensitive model”; JAMA. 2002; 288: 3035-3038), much research has focused on developing treatment algorithms specific to each of three phases of cardiac arrest.
The first phase is known as the “Electrical Phase” and constitutes the first four minutes of a cardiac arrest. During this time, immediate defibrillation should be administered.
The second phase is known as the “Circulatory Phase” and occurs after the first phase for another period of four minutes—that is, four to ten minutes after arrest. During this phase, CPR should be administered before defibrillation in order to increase perfusion and prepare the myocardium for defibrillation by re-oxygenation, thereby increasing the chances of success of the therapy.
The final phase is known as the “Metabolic Phase” and the only available treatments are mild or moderate hypothermia, metabolic therapies or the use of Caspase inhibitors, all of which are only applicable to in-hospital patients.
Studies have shown that survival rates are much lower for patients presenting prolonged ventricular fibrillation (VF). In these cases, immediate defibrillation appears to simply convert the patient's electrocardiogram (ECG) from one non-perfusing rhythm (i.e. VF) to another (i.e. PEA/asystole). It has also been shown that immediate defibrillation in cases of prolonged cardiac arrest would result from countershock-induced injury to ischemic myocardium.
The condition of the myocardium deteriorates rapidly without effective CPR to perfuse the heart muscle and other vital organs. It is widely accepted that, for VF of short duration (less than 4 mins since VF onset), immediate shock therapy is indicated, whereas for VF of long duration (more than 4 mins since VF onset), CPR prior to defibrillation increases the chances of return of spontaneous circulation (ROSC).
It is evident that if the responder had accurate information as to which phase of VF the patient was presenting, they could deliver the most appropriate form of therapy and improve their chances of survival.